Structure and physical properties of the barium niobium sulfides BaNbS₃ and BaNb_0.8S_3-δ

BaNb_0.8S_3-δ and BaNbS₃ were prepared by heating desired amounts of BaCO₃ and Nb₂O₅ under CS₂/N₂ flow. We found that nominal composition of BaNb_0.8S has sulfur defect, and therefore should be written as BaNb_0.8S_3-δ. XANES experiments of Nb K-edges and XPS experiments proved that the oxidation state of Nb is the same in BaNb_0.8S_3-δ and BaNbS₃. To understand their structures, the electronic and physical properties of the BaMX₃ (M = V, Nb, X = S; M = Ta, X = S, Se) phases are compared. In spite of their structural similarity, they exhibit rather different physical properties. It has been known that BaVS₃ is a metal and undergoes a metal to semiconductor transition at 130 K, and this phase shows the ferro antiferromagnetic transition at a lower temperature. However, BaNbS₃ and BaTaS₃ are diamagnetic semiconductors at room temperature. The major factor stabilizing the semiconducting state for BaTaX₃ and BaNbS₃ may be a large spin-orbit coupling rather than a structural distortion.